The present invention relates to an active matrix liquid crystal display device adopting switching elements such as MIM (metal insulator metal), TFT (thin film transistor).
In recent years, a liquid crystal display device which consumes less power and has superior portability is often adopted as a display device of mobile phones. Especially, an STN-LCD (super twisted nematic-liquid crystal display) having a simple structure and of lower cost is widely adopted.
As shown in FIG. 17, the STN-LCD has a glass substrate 101 made of glass and a counter substrate 102 which are opposed via a liquid crystal layer (not shown). Further, a sealing section made of a sealing material for sealing the liquid crystal of the liquid crystal layer is provided so as to enclose the display area between the glass substrate 101 and the counter substrate 102.
The glass substrate 101 includes common lines 103 which also act as pixel electrodes for applying a voltage to the liquid crystal layer. The common line 103 is connected to a reference signal driver via a COM electrode on the glass substrate 101. The counter substrate 102 includes segment lines 104 which also act as pixel electrodes. The segment line 104 is connected to a gradation signal driver via a SEG electrode on the counter substrate 2. Further, the common line 103 and the segment line 104 are orthogonal, and both formed by a transparent conductive film such as ITO.
Further, in a color STN-LCD, the common line 103 is formed on a color filter and on an overcoat which protects the color filter. The overcoat is easy to get a scratch. This may cause the transparent conductive film which is formed on the overcoat to be the common line 103 to cut off even by an indistinct scratch which can be made during manufacturing processes since the common line 103 and the segment line 104 are formed on different substrates. Further, adhesion between the overcoat and the transparent conductive film made of ITO or the like is exceedingly weak in comparison with, for example, adhesion between glass and a transparent conductive film. Therefore, it is almost impossible to re-mount the reference signal driver and/or the gradation signal driver in the case where mounting is failed.
On the other hand, in a small or medium sized panel used for a display of a mobile phone, commonly, an input terminal of the common line 103 is formed on the counter substrate 102. And the input terminal and the common line 103 are electrically connected by transfer technology using conductive particles which are distributed in the sealing section. Hereinafter, this connection part is referred to as xe2x80x9ca transfer sectionxe2x80x9d. By thus electrically connecting the input terminal and the common line 103 via the transfer section, the common line 103 and the segment line 104 can exist on a single substrate (the counter substrate 102).
As described, since the common line 103 electrically transfers to the transparent conductive film made of ITO or the like formed on the counter substrate 102 via the conductive particles distributed in the sealing section, it prevents the cutoff of the common line 103, and also, makes it possible to re-mount the reference signal driver and/or the gradation signal driver in the mounting process. Further, since the input terminal of the common line 103 is formed on the counter substrate 102, the common line 103 and the segment line 104 can exist on a single substrate (the counter substrate 102). This makes it possible to adopt a segment-common integral driver, and make the mounting compact in size.
However, in the foregoing STN-LCD, variation of contact resistance between the adjacent transfer sections is perceived as non-uniform display. Therefore, when assuming the mean distribution volume of the conductive particle is D, and its distribution is "sgr", even when the distribution density of the conductive particle is small like D-5"sgr", it is necessary to prevent contact resistance from being perceived as non-uniform display by conserving the transfer section area as large as possible and increasing the number of the conductive particle in the transfer section.
Here, the following will explain the variation of distribution volume of the conductive particle.
As shown in FIG. 18, when the conductive particles are distributed, the distribution is approximated by a normal distribution around the mean distribution volume D. When the volume of the conductive particle is more than the mean volume D, it is possible to ensure stable connection in the transfer section. In contrast, when the volume of the conductive particle is less than the mean volume D, connection in the transfer section, in other words, resistance in the transfer section varies depending on the distribution volume of conductive particle. Table 1 shows separation from the mean distribution volume D and probability of existence, regarding to the distributed conductive particles. (continued)
If assuming that the minimum particle density to prevent the contact resistance in the transfer section from being perceived as non-uniform display is D0, Table 1 shows that poor contact occurs at a rate of 0.15% when D0=Dxe2x88x923"sgr". Namely, when a panel has 160 transfer sections, at least one in 4.2 panels will show poor contact. Similarly, as shown in Table 1, poor contact occurs at a rate of 5.73exe2x88x925% when D0=Dxe2x88x925"sgr". In this case, when a panel has 160 transfer sections, at least one in 10908 panels will show poor contact. Determination of the mean distribution volume D is important in the distribution of the conductive particle, because distribution "sgr" is automatically determined by the mean distribution volume D, even though the distribution "sgr" can be adjusted to some extent by using an automatic stirring device for stirring the sealing material.
The permitted limit of the STN-LCD for the contact resistance variation in the adjacent transfer sections becomes smaller, as the STN-LCD has high-precision (256 colorsxe2x86x924096 colors) and multi-gradation displays (6500 colors). Further, as the line width of sealing section becomes narrower in accordance with high-precision and narrower frame (narrower non-display area), the area of the transfer section becomes smaller. Accordingly, it is difficult to apply the technology of electric transfer using the conductive particles distributed in the sealing section to the STN-LCD in terms of high-precision, multi-gradation displays, and narrower frame (narrower non-display area) which are major factors for a mobile phone in next-generation.
Meanwhile, liquid crystal display devices of active driving type in which a switching element such as MIM or TFT of counter source structure have been proposed as a liquid crystal display device (LCD) having a simple structure like the STN-LCD. These liquid crystal display devices are more suitable for high-precision, multi-gradation displays, and narrower frame which are major factors for a mobile phone in next-generation, in comparison with the STN-LCD.
FIG. 19 shows an example of an equivalent circuit of the arrangement in a conventional active matrix liquid crystal display device. In this liquid crystal display device, pixel electrodes 111 are formed in a matrix manner on a transparent substrate which will be an active matrix substrate. Further, on the transparent substrate, a TFT 112 which is a switching element is provided for each pixel electrode 111. In each TFT 112, the pixel electrode 111 is connected to a drain electrode. And in the TFTs 112 which are horizontally (in a column direction) aligned in a display screen, respective gate electrodes are connected to a same reference scanning line 113, and also are connected to the same data line 114 in the TFTs 112 vertically (in a row direction) aligned in the display screen. Namely, the respective scanning lines 113, and the respective data lines 114 in the above directions are orthogonally disposed while surrounding the pixel electrode 111.
With the foregoing arrangement, the TFT 112 is controlled so as to turn on/off in response to the gate signal which is supplied via the scanning line 113. And when the TFT 112 is on, a data signal (display signal) is sent to the pixel electrode 111 via the data line 114.
Further, each drain electrode of the TFTs 112 is individually connected to the pixel electrode 111, and an electrode which forms an accumulation capacitor 115, and the other electrode opposite to the former electrode via an insulation layer is connected to a reference signal line 116. The accumulation capacitor 115 holds a voltage which is applied to the liquid crystal layer.
In the active matrix liquid crystal display device thus described, the liquid crystal layer is caught between the active matrix substrate and the counter substrate which is opposite to the active matrix substrate.
However, in the active matrix liquid crystal display device shown in FIG. 19, poor connection due to the cutoff is likely to occur at a crossing section of the scanning line 113 and the data line 114 which are orthogonally disposed on a single substrate. This decreases the yield, and increases the manufacturing cost.
This being the case, in order to solve these problems, a structure having data lines disposed on a counter substrate (hereinafter, referred to a counter matrix structure) has been conventionally proposed. An example of the arrangement of the counter matrix structure is shown in FIG. 20, FIGS. 21(a) through 21(d).
In this type of liquid crystal display device, pixel electrodes 124 are provided in a matrix manner on a glass substrate 120, and TFTs 121 are formed on the respective pixel electrodes 124. The drain electrode (or the source electrode) of each TFT 121 is connected to the pixel electrode 124, and the gate electrode is connected to a same scanning line 122 among the TFTs 121 which are horizontally (in a column direction) aligned in a display screen. These arrangements are the same as those of the liquid crystal display shown in FIG. 19. However, in contrast, the source electrode (or the drain electrode) of each of the TFTs 121 which are horizontally (in a column direction) aligned in the display screen is connected to a same reference signal line 123 instead of connected to the data line 114 where a data signal is applied, unlike the liquid crystal display device shown in FIG. 19. And gradation signal lines 126 are orthogonally disposed to scanning lines 122 of the glass substrate 120 on a counter substrate 125 which is opposite to the glass substrate 120 via a liquid crystal layer. Note that, in this arrangement, each gradation signal line 126 also acts as a counter electrode at the portion opposite to the pixel electrodes 124.
In the active matrix liquid crystal display device having the counter matrix structure thus described, since the crossing sections of the scanning line 122 and the gradation signal line 126 do not exist on a single substrate, the foregoing problems: decreases of the yield and reliability caused by poor connection due to the cutoff can be solved.
However, according to the arrangement shown in FIG. 20, FIGS. 21(a) through 21(d), since the scanning line 122 and the gradation signal line 126 are formed on respective substrates, the gradation signal line 126 made of a transparent conductive film of ITO or the like formed on an overcoat 131 (see FIG. 21(d)) may cut off even by an indistinct scratch which can be made during manufacturing processes. Further, adhesion between the overcoat 131 and the gradation signal line 126 is exceedingly weak in comparison with, for example, adhesion between the glass substrate 120 and the gradation signal line 126 made of transparent conductive film. Therefore, it is almost impossible to re-mount these liquid crystal display elements during a mounting process in the case where mounting is failed.
This being the case, a transparent conductive film (gradation signal line 126) made of ITO or the like is formed on a glass (glass substrate 120) by selectively removing the overcoat 131, to which having photosensitivity has been given, of the mounting part. This will solve the foregoing problems; however, the transmission rate of the overcoat 131 decreases due to its photosensitivity, and the transmission rate or the reflectance of the liquid crystal panel decreases. Further, an extra process for selectively removing the overcoat 131 is required. Moreover, the transparent conductive film of ITO or the like may cut off because of the difference in level between the portion having the overcoat 131 and the portion having no overcoat 131.
As shown in FIG. 21(b), a counter electrode 128 and an input terminal 127 are connected to the gradation signal line 126 on the counter substrate 125. Further, in addition to the arrangement shown in FIG. 20, a gate insulation film 135 (see in FIG. 21(d)), an input terminal 130 connected to the gradation signal line 123, and an input terminal 129 connected to the scanning line 122 (see in FIG. 21(a)) are formed on the glass substrate 120. Further, a sealing section 134 which seals liquid crystal has spacers 136, and its cell thickness is held by the diameter of the spacer 136.
Furthermore, since the scanning line 122 and the gradation signal line 126 are formed on respective substrates, the input terminal 127 and the input terminal 129 are also formed on respective substrates. Therefore, as shown in FIG. 21(c), the liquid crystal element becomes bulky when TAB 133 is mounted, and it is impossible to adopt a compact mounting such as COG.
It is an object of the present invention to provide a liquid crystal display device with smaller frame and the compact mounting in size without causing poor connection due to cutoff.
In order to attain the foregoing object, a liquid crystal display device according to the present invention includes a switching element substrate having switching elements, a counter substrate opposite to the switching element substrate, a liquid crystal layer formed between the substrates, a sealing section having conductive particles and provided so as to enclose a display area between the substrates for sealing liquid crystal constituting the liquid crystal layer, first signal wiring provided on one of the substrates for controlling the switching element, second signal wiring opposite to the first signal wiring and provided between the substrates for applying a voltage to the liquid crystal layer, and at least one transfer section for electrically connecting the first signal wiring or the second signal wiring and the substrate opposite to the first signal wiring or the second signal wiring via the conductive particles.
With the foregoing structure, the first signal wiring or the second signal wiring, and a substrate opposite to the first signal wiring or the second signal wiring, are electrically connected via the conductive particles by the transfer section. Namely, the first signal wiring and the substrate opposite to the first signal wiring, or the second signal wiring and the substrate opposite to the second signal wiring are electrically connected. Commonly, active driving has a wider permitted limit of contact resistance variation in the adjacent transfer sections than that of passive driving. Therefore, it becomes possible to provide a liquid crystal display device of lower cost since active driving requires less volume of the conductive particles than that of passive driving.
Additional objects, features, and strengths of the present invention will be made clear by the description below. Further, the advantages of the present invention will be evident from the following explanation in reference to the drawings.